1. Field of the Invention
The present invention relates to a check valve with an valve opening device for use in supplying and discharging a pressurized oil with respect to a hydraulic cylinder, for example in a hydraulic clamp apparatus adapted to secure a work to a machine tool.
2. Prior Art
As such a check valve with a valve opening device has been known the one disclosed in the Japanese Utility Model Publication No. 1988 - 24304 previously proposed by the inventor of the present invention. This check valve has the following basic construction as illustrated in FIGS. 11 and 12.
An inlet port 122 is opened in one end surface of a check valve chamber 121 within a valve box 120 and an outlet port 128 is opened in another surface thereof besides the inlet port 122. A check valve member 132 is inserted into the check valve chamber 121 with being resiliently urged by means of a check spring 133 for valve closing to a check valve seat 134 formed in the peripheral wall of the inlet port 122. A valve opening member insertion port 123 is so formed as to face the inlet port 122 of the check valve chamber 121, and a valve opening member 136 is slidably and oil-tightly inserted into the valve opening member insertion port 123. Herein, the valve opening member 136 is adapted to be changed over between a valve opening position F where the check valve member 132 is moved for valve opening by advancing the valve opening member 136 toward the check valve chamber 121 and a valve opening cancellation position E where the valve opening member 136 is retreated outside the inlet port 122 from the check valve chamber 121. The right end portion of the valve opening member 136 is so disposed as to face the check valve member 132 from the side of the inlet port 122, and the left end portion thereof is so disposed as to face an actuation portion 144 of an actuator shaft 143. The actuator shaft 143 is adapted to be turned by means of a manipulation lever 145 in the same direction. The manipulation lever 145 is adapted to be changed over between a valve opening posture B (refer to a figure depicted by the alternate long and short dash line) where the valve opening member 136 is changed over to the valve opening position F on the right side through the actuator shaft 143 and a valve opening cancellation posture A (refer to a figure depicted by the alternate long and two short dashes line) where the valve opening member 136 is allowed to be changed over from the valve opening position on the right side to the valve opening cancellation position E on the left side.
In the portion from the manipulation lever 145 to the valve box 120 through the actuator shaft 143 and the valve opening member 136 there is provided a valve opened state holding means 138 for holding the valve opening member 136 at the valve opening position on the right side. And, the valve opening member 136 is adapted to be automatically returned from the valve opening position on the right side to the valve opening cancellation position E on the left side when it is pushed toward the actuator shaft 143 from the condition in which the valve opening member 136 is held at the valve opening position on the right side by means of the valve opened state holding means 138, by supplying a pressurized oil to the inlet port 122.
In the aforementioned basic construction, as illustrated in FIGS. 11 and 12, conventionally the valve opened state holding means 138 is provided in both the valve box 120 and the actuator shaft 143, and the actuator shaft 143 is so disposed that the axis thereof is eccentric to the axis of the valve opening member 136.
When the check valve member 132 is returned from the valve opened state to the illustrated valve opening cancellation state, the valve opening member 136 is changed over from the valve opening position to the illustrated valve opening cancellation position E by means of a hydraulic pressure applied to the valve opening member 136 from the inlet port 122 at the time of pressure supply, so that the actuation portion 144 of the actuator shaft 143 is pushed through the valve opening member 136. Thereby, the actuator shaft 143 is turned by means of a turning moment produced by the aforementioned eccentricity and the manipulation lever 145 is automatically returned from the valve opening posture B to the valve opening cancellation posture A.
There are, however, the following problems associated with the aforementioned prior art.
Since there are four resistances of a sliding resistance at a sealing portion 137 of the valve opening member 136, a sliding resistance at a contact portion between the actuation portion 144 of the actuator shaft 143 and the valve opening member 136, a lock cancellation resistance at the valve opened state holding means 138 and a turning resistance at the actuator shaft 143 as a resistance subjected to the valve opening member 136 when the valve opening member 136 is changed over from the valve opening position on the right side to the valve opening cancellation position E on the left side by means of the hydraulic pressure in the inlet port 122, the resistance against the valve opening member 136 becomes large. Accordingly, a check valve 107 with the valve opening device is not suitable for a low pressure application.
Then, since the turning resistance at the actuator shaft 143 irregularly changes due to intrusion of foreign substances, generation of rusts, lack of lubrication and so on, the pressure in the inlet port 122 becomes unstable when the valve opening member 136 is changed over from the valve opening position to the valve opening cancellation position E.
Further, accompanied with the unstable changeover of the aforementioned valve opening member 136, it is apprehended that the manipulation lever 145 is suddenly changed over from the valve opening posture B to the valve opening cancellation posture A.